The properties of granular polymers, polymer powder recovered from a polymerization reactor, substantially depend upon the properties of the catalysts used in their preparation. In particular, the choice of the shape, size, size distribution, and other morphological properties of the solid catalysts is important to ensure operability and commercial success. This is particularly important in gas phase and slurry polymerizations. A successful catalyst composition should be based on a procatalyst particle having good mechanical properties including resistance to wear, abrasion and shattering during the polymerization process, thereby imparting good bulk density and uniformity to the resulting polymer product. Equally important are procatalyst compositions that produce such polymer products in high catalyst efficiency.
Spray-drying is a well known technique for preparing solid Ziegler-Natta polymerization procatalysts. In spray-drying, liquid droplets containing dissolved and/or suspended materials are ejected into a chamber under drying conditions to remove solvent or diluent leaving behind a solid residue. The resulting particle size and shape is related to the characteristics of the droplets formed in the spraying process. Structural reorganization of the particle can be influenced by changes in volume and size of the droplets. Depending on conditions of the spray drying process, either large, small, or aggregated particles can be obtained. The conditions may also produce particles that are compositionally uniform or contain voids or pores. The use of inert fillers in forming spray-dried particles can help control shape and composition of the resulting particles.
Numerous spray-dried olefin polymerization procatalysts containing magnesium and titanium and production processes for making and utilizing them have been reported, including for example, U.S. Pat. Nos. 6,187,866; 5,567,665 and 5,290,745, each of which is incorporated herein by reference. Generally, such compositions have been produced in the form of substantially spheroidal solid procatalyst particles having average particle diameters from 1 to 100 microns, depending on the intended end use. Porosity and cohesive strength of the particles can be adjusted by the use of fillers, such as silica, and binders, such as polymeric additives. Generally, solid rather than hollow particles are desired due to greater structural integrity of the resulting particles.
Known spray-dried olefin polymerization catalysts are characterized in the use of flammable solvents to dissolve the active components. All commercially viable spray drying processes that utilize flammable solvents are of the “closed cycle” type in which the solvent utilized is recovered for reuse and the inert gas used in the spray drying process is recycled. Generally the solvent recovered will be reused in the preparation of additional feedstock, improving the efficiency of the process as well as avoiding the disposal of large amounts of solvent.
While both environmentally and fiscally sound, such recycle of solvent can pose challenges if impurities are present in the feedstock solution. In general, any volatile compound in the feedstock will be recovered in the recycled solvent and accumulate. Thus, impurities such as acidity will not only cause potential corrosion in the production of the feedstock but will also collect in the recycle solvent, potentially damaging the spray drying operation. Also, a buildup of impurities can be deleterious to the final spray dried catalyst product.
These spray dried polymerization procatalysts can be particularly valuable in multi-reactor operation in which two or more reactors are connected in series to produce resins with fractions having large separations in molecular weight and/or density. Due to these very large differences in desired molecular weight, the reaction conditions in the separate reactors may be radically different as well. In particular, the low molecular weight reactor(s) will generally have high hydrogen concentrations, in some cases 30 to 70 mol % of the reactor gas may be hydrogen. With these very high hydrogen levels and the presence of aluminum alkyl cocatalysts, even very small amounts of impurities such as Fe, Ni or Cr that are present in a form that can be converted from oxides or halides to a zero valent state will result in formation of hydrogenation catalysts that convert, in particular, ethylene monomer to ethane. Thus even small amounts of corrosion occurring in the spray drying stage can have a significant negative impact on the entire process.
Additionally, the generation of ethane from ethylene is both wasteful of monomer (monomer not being incorporated into the desired polymer and results in buildup of an inert in the low molecular weight reactor (i.e. ethane) that subsequently limits the amount of monomer in the reactor, thus negatively impacting catalyst activity. Reduced catalyst activity will also result in smaller size particles and additional levels of fine particles.
Despite advances in the art, there still remains a need for a method to produce Ziegler-Natta procatalysts having improved performance properties. In particular, procatalyst compositions that can produce resins with improved polymer properties are particularly important. In addition, there is a need for procatalyst compositions with increased resistance to shattering and thus, reduced generation of polymer fines.
Polymer fines are undesirable due to buildup in the polymerization equipment, thereby causing problems with bed level control and entrainment in the cycle gas leading to equipment failure, impaired operability, and reduced efficiency. High levels of fines can also cause problems in downstream handling of the polymer once it exits the polymerization system. Fines can cause poor flow in purge bins, plug filters in bins, and present safety problems. Such problems make elimination or reduction of polymer fines important to commercial operations, especially gas-phase polymerization processes.
Thus, it is preferable to have catalyst systems that produce improved polymer properties, can be successfully spray dried to produce the procatalyst particles and produce procatalyst particles of strength and solidity that resist fragmentation and fines generation.
In a multiple series reactor system, where the composition of the polymers produced in the separate reactors is widely variable, the presence of polymer fines is particularly harmful to continuous and smooth operation. This is due to the importance of precise bed level control, in as much as the product properties of the final polymer are strongly influenced by the relative amount of polymer produced in each reactor. If the bed weights are not precisely known, it is difficult to properly control the final product properties.
With respect to the preparation of polyethylene and other ethylene/a-olefin copolymers, it is preferred to produce polymer in separate reactors with both large molecular weight differences and relatively large differences in incorporated comonomer. To produce final polymers with the best physical properties, it is preferred to have one of the reactors produce a polymer with high molecular weight and incorporating a majority of any comonomer present. In the second reactor, a low molecular weight portion of the polymer is formed which may also have comonomer incorporated, but normally in an amount less than that incorporated in the high molecular weight portion. In some instances, the low molecular weight portion of the polymer is a homopolymer.
Depending on the order of production of the different polymers in the multiple reactor system (that is production of high molecular weight polymer first and lower molecular weight polymer second or vice versa), the fines from known catalysts will tend to have significantly different polymer properties than the bulk of the polymer granules. This is believed to be due to the fact that the fines also tend to be the youngest particles in the reactor and hence they do not achieve conformation to the final product properties before transiting to the second reactor in series. Such a difference in the fine and bulk polymer properties leads to challenges in compounding the polymer into pellets for end-use.
In particular with known catalysts, the fines are normally of significantly different molecular weight or branching composition compared to the bulk polymer. Although the particles of both the bulk material and the fines will melt at roughly the same temperature, mixing is hampered unless the products have a similar isoviscous temperature (that is the temperature at which the melt viscosity of the two products is essentially the same). The polymer fines, which tend to have significantly different molecular weight and isoviscous temperature than those of the bulk polymer, are not readily homogeneously mixed with the bulk polymer, but rather form segregated regions in the resulting polymer pellet and can lead to gels or other defects in blown films or other extruded articles made therefrom.
Thus, generation of polymer fines is preferably avoided, especially for gas phase olefin polymerization processes and, in particular, for staged or series reactor systems in which control of polymer composition is achieved by control of the relative amount of polymer produced in the multiple reactors.
Accordingly, there is a need to minimize polymer fines in an olefin polymerization process. It is also desirable to produce polymers with improved properties, particularly broader molecular weight distributions that are suitable for blow molding and other extrusion processes.